It is well-known that vehicles having spaced apart drive wheels or wheel-sets powered by a single engine through a differential mechanism, are problematic when one of the differentially driven wheels or wheel-sets loses traction. Conditions which give rise to a loss of traction commonly exist in construction sites and other off-road locations. A vehicle having one of two differentially driven wheels or wheel-sets on a slippery surface and the other on a surface providing good traction is often unable to move, owing to the fact that the differential directs full engine power to the wheel having no traction. The result is a slip condition in which the wheel having no traction rotates at higher than normal speed and the wheel having traction remains stationary.
To alleviate such problems, various mechanical anti-spin devices have been developed and placed in commercial use. Such mechanical devices have been proven to have various problems, especially during cornering of a vehicle. Some devices fail to accommodate the normal wheel speed differential which arises during a turn, causing excessive tire wear owing to the dragging of the radially outer wheel or wheel-set.
Other devices drive only the slower wheel in a turn, making the vehicle hard to steer and applying excessive torque to the wheel being driven, often causing failure of the final drive.
An alternative approach involves the provision of separately actuatable drive wheel brakes. An operator selectively applies a braking force to the spinning or slipping wheel, and effects a balancing of power through the differential mechanism. The application of the braking force to the slipping wheel simulates increased traction and results in a more even distribution of power between the differentially driven wheels. This approach is commonly used on farm vehicles.
A more sophisticated approach to the just described system, utilizes electronics to supply the braking force to the slipping or spinning wheel. An effective example of this approach is described in U.S. Pat. No. 4,344,139, issued to Miller et al. on Aug. 10, 1982, and assigned to the assignee of this invention. Miller discloses an apparatus for applying a proportionally varying braking force to the wheel which loses traction, during a slip control time period. A slip signal is produced corresponding to any difference between the rotational velocity of the differentially driven wheels, and the slip signal is compared with a predetermined reference signal. In response to the slip signal exceeding the reference signal, the system selectively applies a braking force to the faster turning wheel. The braking force is modulated proportionally according to the degree of slip represented by the slip signal.
One problem with the various prior control systems involves the normal difference in rotational velocities of the inner and outer vehicle wheels encountered while cornering. To avoid this problem, a fully automatic system, such as that described by Miller, must establish the reference signal at a level higher than the maximum slip signal that is produced solely in response to cornering the vehicle. Thus, the automatic control is inhibited for small values of wheel slip. Other prior systems have eliminated the problem completely by providing only manual operation. These controls rely on the operator to activate the anti-slip control when he senses the need to do so. Such manual systems necessarily eliminate much of the advantage of an anti-slip control. A further problem with the fully automatic systems is the inability of the operator to determine the operational status of the anti-spin control prior to encountering an actual slip condition.
The present invention is directed to overcoming one or more of the problems as set forth above.